1. Field of the Invention
This invention pertains generally to NMR and MRI, and more particularly to a method and apparatus for NMR and MRI in inhomogeneous magnetic fields.
2. Description of the Related Art
Modern NMR spectroscopy and MRI require highly homogeneous strong magnetic fields. This leads to physically large and expensive super-conducting magnets. In NMR, highly homogeneous magnets (superconducting, permanent magnets or electromagnets) are used in order to detect the Free Induction Decay (FID) of the nuclear magnetization. The frequency and the intensity of this signal is proportional to the applied magnetic field. In an inhomogeneous magnetic field the magnetization dephases because of different precession frequencies in different parts of the sample. Field inhomogeneities lead to line broadening, poor signal-to-noise ratio, and loss of spectral information. For this reason special effort is made in the construction of electromagnetic coils (shims) which are used in order to render the original magnetic field as homogeneous as possible over the detected sample volume. The excitation and detection of the FID is made using a radio-frequency (RF) coil also having very good homogeneity. RF inhomogeneities have been traditionally seen as sources of imperfection in excitation, inversion and decoupling, leading to spectral artifacts and poor signal-to-noise ratios.
The following publications provide additional background information and are incorporated herein by reference in their entirety:
High-resolution NMR of anisotropic samples with spinning away from the magic angle, 377, 333-339 (2003), Chem. Phys. Lett (D. Sakellariou, C. A. Meriles, R. W. Martin and A. Pines).
Broadband Phase Modulation by Adiabatic Pulses, Journal of Magnetic Resonance, 164, 177181 (2003), (C. A. Meriles, D. Sakellariou and A. Pines).
Variable Rotation Composite Pulses for High Resolution Nuclear Magnetic Resonance Using Inhomogeneous Magnetic and Radiofrequency Fields Chem. Phys. Lett., 363, 25-33 (2002) (Dimitris Sakellariou, Carlos Andres Meriles, Adam Moule and Alexander Pines).
2-Dimensional High-Resolution NMR Spectra in Matched B0 and B1 Field Gradients, J. Magn. Reson. 156 (1), 146-151 (2002) (Henrike Heise, Dimitris Sakellanou, Carlos Meriles, Adam Moule and Alexander Pines).
Resolved magic-angle spinning of anisotropic samples in Inhomogeneous Fields, Chem. Phys. Lett. 358 (5,6), 391-395 (2002) (Carlos Andres Meriles, Dimitris Sakellariou and Alexander Pines).
Nuclear Magnetic Resonance in inhomogeneous magnetic fields, J. Magn Reson., 145, 246-258, (2000), (F. Balibanu, K. Hailu, D. E. Demco, and B. Blumich).
Approach to High-Resolution Ex-Situ NMR Spectroscopy, Science, 293, No. 5527, 82-85 2001 (Carlos Meriles, Dimitris Sakellariou, Henrike Heise, Adam Moule, Alexander Pines).
High resolution NMR in Inhomogeneous fields, J. Magn. Res., 145, 246-258, (2000), (J. J. Balbach, M. S. Conradi, D. P. Cistola, C. Tang, J. R. Garbow and W. C. Hutton).
Homogeneous NMR Spectra in Inhomogeneous Fields, Science, 272, 92-96, (1996), (S. Vathyam, S. Lee and W. S. Warren).
Measurement of High-Resolution NMR spectra in a inhomogeneous magnetic field, J. Am. Chem. Society, 109, 7579-7581, (1987) (L. D. Hall, T. J. Norwood).
A SHARP Method for High Resolution NMR of Heteronuclear Spin Systems in Inhomogeneous Fields, J. Magn. Reson., 63, 431-437 (1985) (M. Gochin, D. P. Weitekamp, and A. Pines).
Total Spin Coherence Transfer Echo Spectroscopy, J. Chem. Phys. 79, 5301-5310 (1983) (J. R. Garbow, D. P. Weitekamp, and A. Pines).
High Resolution NMR in Inhomogeneous Magnetic Fields: Application of Total Spin Coherence Transfer Echoes, J. Am. Chem. Soc., 103, 3578-3579 (1981) (D. P. Weitekamp, J. R. Garbow, J. B. Murdoch, and A. Pines).